Proton formation and diffusion in amorphous SiN x :H
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Proton formation and diffusion in amorphous SiNx:H H.F.W. Dekkers1, V. Prajapati2, S. Van Elshocht1 and E. Vancoille1 1 FPS, imec, Kapeldreef 75, 3001 Leuven, Belgium 2 SSET, imec, Kapeldreef 75, 3001 Leuven, Belgium. ABSTRACT In this work the release of atomic hydrogen from SiNx:H films is investigated. Thermal treatment as well as UV-illumination induces the formation of H2, increasing the tensile stress in the film. N-rich SiNx:H films release hydrogen only by UV-illumination, indicating involvement of charge trapping. Ab initio calculations show a possible reaction path for the release and diffusion of protons that also explain the diffusion of hydrogen into Si substrates. INTRODUCTION The release of bonded hydrogen from amorphous silicon nitride (SiNx:H) films can provide additional functionality in semiconductor devices. For example, SiNx:H etch-stop-layers, deposited on top of MOSFETs can be used to induce tensile strain in the gate channels, increasing their charge carrier mobility [1]. This strain is caused by tensile stress in the film due to cross linking of Si–H and N–H sites via the reaction: Si–H + N–H → Si–N + H2
(1).
In this reaction, hydrogen is released in molecular form [2]. To avoid high thermal budgets, UV-illumination can be used to enhance this reaction at temperatures below 400 °C. Another application is the hydrogenation of defects in Si substrates on which SiNx:H is deposited as an anti-reflection coating. This is an important step in the manufacturing process of multi-crystalline silicon (mc-Si) solar cells. Despite the fact that the exact nature of defects in mc-Si, subject to this passivation, is not clearly identified [3], significant improvements in minority carrier lifetimes in mc-Si substrates are observed after thermal treatment [4]. Several experiments demonstrated fast diffusion of hydrogen through crystalline Si samples of a few hundred micrometers thickness [5,6]. Fast diffusion is only possible when hydrogen is in an atomic form [7]. Therefore, atomic hydrogen must become available when released from SiNx:H. Photoconductance measurements on both N-rich and Si-rich material show conduction under sub-band gap illumination. This indicates a large density of band gap states that are able to trap charge carriers which are injected by UV illumination. The capture of holes by N-H bond was earlier [8] used to explain the relatively easy way of H2 release, according: Si–H + N+–H → Si• + N+=H2 → N–Si + H2 + h+
(2).
In this work the mechanism of hydrogen release is further explored experimentally and theoretically.
EXPERIMENTAL DETAILS SiNx:H films were deposited on double side polished 300 mm Cz-Si wafer in a dual chamber Producer SA system of Applied Materials. Precursor gases are SiH4, NH3 and N2. The density of the SiNx:H can be tuned from 2.2 to 2.9 g/cm3 by varying the pressure 9 - 1.5 Torr and adding RF powers of both low frequency (150kHz) and high frequency (13.56MHz). The Si/N ratio for the lowest density films has been optimized to obtain the highest stress value after th
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